This invention is directed to an improved use of zinc in rechargeable electrical storage batteries incorporating an aqueous alkaline electrolyte and wherein the zinc is used as the negative electrode, i.e. anode. Typical positive electrodes (cathodes) used in conjunction with these zinc anodes include nickel oxide, AgO, Ag.sub.2 O, MnO.sub.2, HgO, PbO.sub.2 and the like.
The use of zinc electrodes in rechargeable batteries has long been recognized as a means of obtaining high energy density and high battery voltage compared to less electronegative anode materials such as cadmium, iron or lead. Zinc also has the advantage of being relatively light in weight, relatively abundant and relatively low in cost.
As is well known to those involved in the field, zinc electrodes where used in an alkaline electrolyte battery, have a habit of forming adendrites or "trees" during repeated charge/discharge cycling. This significantly shortens the useful cell life. If left unchecked, these dendrites or tree-like crystalline zinc growths will quickly bridge across to the positive electrode, usually causing battery shorting in a very few use cycles.
Associated with the dendrite problem at the anode is the so-called shape change phenomenon. This is a shifting of the zinc on the anode current collector that can cause a serious reduction in battery efficiency. The dendrite and shape change effects are considered by many to be closely interrelated, and are thought to be linked to the solubility of zinc in the alkaline electrolyte solution.
Cell shorting by dendrite growth can be slowed down by the inclusion of microporous membranes as part of the separator system used between the electrodes. Other techniques such as intermediate electrodes that oxidize the dendrites on contact and electrolyte additives that act to prevent zinc from going into solution have been disclosed. Specially shaped anodes have been found helpful in some cell configurations for reducing shape change. While cycle life can sometimes be increased by these and other methods, this is invariably accomplished at the expense of some other factor. Perhaps primary among these are increased internal cell resistance, increased battery size and weight and increased battery cost. Special electrical charging systems have been reported to slow dendrite growth or change their crystalline form.
None of the approaches reported to date, when used singly or in combination with each other, seem to have adequately solved the zinc anode problem for long cycle life rechargeable storage batteries. A viable solution to this problem is currently being sought by many investigators. The closest prior art patents of which applicant is aware are as follows:
U.S. Pat. No. 3,617,384
U.S. Pat. No. 3,672,996
U.S. Pat. No. 3,785,868
U.S. Pat. No. 3,970,472